So far, fermentations requiring an high rate of oxygen input, such as high cell density fermentation processes disclosed in U.S. Pat. No. 4,414,329 by Wegner, have been generally conducted in fermentors equipped with agitation means and mechanical foam breakers. The patent literature disclosing such fermentors includes: U.S. Pat. No. 4,670,397, U.S. Pat. No. 4,752,564, U.S. Pat. No. 4,883,759, U.S. Pat. No. 4,373,024. Agitated fermentors are adapted with difficulty for supplying the very high rate of oxygen input necessary to carry out high cell density fermentation processes in large scale plants. Several reasons, including the massive mechanical foam breakers required, made not commercially feasible to increase their size beyond certain limits. Stirrers and mechanical foam breakers used in high cell density fermentations require very high energy input, which is both costly and an operation and maintenance problem in large fermentors. Problems related to the mechanical design of drive shaft and bearings for transmitting the enormous torque arise. In addition, the seals associated with the stirrer and the foam breaker are often difficult to clean and sterilize; in many instances they create cleaning and asepsis problems. Consequently, there is a continuing need for improved fermentors of even larger capacity than those built to date, able of conducting aerobic fermentation processes at high cell densities. At the same time, fermentors which do not require moving parts for effective foam control, simple in construction, with low manufacturing cost and easy to maintain, together with high oxygen transfer rates at low power consumption, would provide sensible advantages. Fermentors with non mechanical foam control devices are very desirable. The advantages are: improved maintenance of sterility because no shaft is needed, construction of larger fermentors is possible because the design is not limited by the massive foam breaker required in large fermentors, lower initial cost and reduced maintenance. The present invention is intended to provide a fermentor and a fermentation method which overcome several shortcomings of fermentors and fermentation methods of the current technology. Particularly, this invention provides an improved fermentation method and a fermentor for carrying out fermentation processes with controlled gas holdup, while simultaneously preventing the formation of a foam layer by an hydrodynamic approach.